Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into an intake port (port fuel injection). The fuel injectors may operate over a range of pulse widths with the amount of fuel injected into the cylinders decreasing with decreasing pulse width. Further, the fuel injectors may have a threshold (e.g., lower threshold) pulse width at which the pulse width and amount of fuel injected may not be further reduced. Under certain operating conditions, when engine airflow is lower and/or fuel pressure in the fuel system is higher, operation of the fuel injectors at the threshold pulse width may result in a rich air-fuel ratio of an exhaust gas. The rich exhaust gas air-fuel ratio may result in reduced exhaust aftertreatment efficiency and increased exhaust emissions.
One example approach for addressing fuel injector operation at a threshold pulse width is shown by Cullen et al. in U.S. Pat. No. 6,273,060. Therein, engine airflow is controlled to maintain an exhaust air-fuel ratio at stoichiometry when fuel injector operation limits are reached. These limits include operation of the fuel injectors at a minimum, or threshold, pulse width. When the fuel injectors are not being operated at the threshold pulse width, engine airflow may be controlled to provide a desired engine torque.
However, the inventors herein have recognized potential issues with such methods. For example, while adjusting airflow to maintain stoichiometry during fuel injector operation at the threshold pulse width may decrease exhaust emissions, engine torque output may not be maintained at a requested level. In some examples, increasing engine airflow to maintain stoichiometry may include increasing airflow to a higher level than required by a torque demand. As such, engine torque output may be greater than requested by the vehicle operator.
In one example, the issues described above may be addressed by a method for adjusting engine airflow and engine actuators in response to fuel injector operation. Specifically, during operation of a fuel injector at a threshold pulse width, engine airflow may be increased to maintain an exhaust gas mixture near stoichiometry while adjusting an engine actuator to maintain torque. In this way, exhaust emissions may be reduced while maintaining requested engine performance.
As one example, an engine controller may increase engine airflow, during operation of a fuel injector at a threshold pulse width, to maintain an exhaust gas mixture at stoichiometry. Specifically, in response to operation of a fuel injector at the threshold pulse width, when an air-fuel ratio of an exhaust gas is less than a stoichiometric ratio, the controller may increase engine airflow greater than requested by a vehicle operator, without increasing engine torque by adjusting an engine actuator to maintain torque. Increasing engine airflow may include increasing an opening of an intake throttle, wherein the amount of opening may be based on the air-fuel ratio of the exhaust gas. In one example, the controller may increase an amount of a spark retard to compensate for the increased engine airflow, thereby maintain engine torque at a requested level. Additional or alternative engine actuator adjustments may be made to maintain torque, including adjusting alternator loading, a variable valve timing, a variable valve lift, and/or an opening of a wastegate.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.